The present invention relates to a method and apparatus for writing a servo track on a storage medium.
Information for systems such as data processing systems is typically stored on storage media. Particular use is made of storage disks such as magnetic disks, opto-magnetic disks, and the like. One type of magnetic disk arrangement is a so-called xe2x80x9chead disk assemblyxe2x80x9d which is intended normally to be permanently fixed in a data processing system; the head disk assembly includes the magnetic disk medium itself and the associated read and write head or heads which write data to the disk and read data from the disk. Another type of magnetic disk is of a type known as xe2x80x9cremovable mediaxe2x80x9d which normally consists of a magnetic disk medium in a protective plastics case which can be used to transfer data between data processing systems by physical transfer of the disk itself from one machine to another.
In a known method of manufacturing storage media such as hard disk drives, a head disk assembly consisting of the head(s), the disk or disks, the motor and arm electronics, is mounted in a mastering station known as a servo-writer. The servo-writer writes a pattern of magnetic information (the xe2x80x9cservo track patternxe2x80x9d) onto the disk. The servo track pattern becomes the master reference which is used by the disk drive during normal operation in order to locate the tracks and sectors on the disk for data storage and retrieval.
In order to be able to write the servo tracks onto the correct positions on the disk during manufacture of the disk, in one commonly used process, a (usually temporary) clock track is written onto the disk to serve as a timing reference during writing of the servo pattern. Conventionally, a separate clock head is used to write the clock track onto the disk and to read the clock track from the disk so that the servo tracks can be phase aligned with respect to each other. However, the use of a dedicated clock track writing head is an expensive addition to the manufacturing process and further requires that the servo tracks be written in a clean room because the clock track writing head has to be inserted into the open (unsealed) head disk assembly. Moreover, in practice the clock heads have to be replaced on a daily basis because of damage which occurs during use.
U.S. Pat. No. 5,485,322 discloses a method and system for writing a clock track on a storage medium using an internal recording head of a hard disk drive. A timing pattern is generated on the storage medium with the internal recording head and a radial positioning value used in radially positioning the internal recording head is determined. The servo pattern is written at the locations determined by the generated timing pattern and radial positioning value. In effect, in this prior art system, the clock pattern is written in an iterative manner across the disk. However, there is a problem in that this method can introduce phase and frequency differences in a clock pattern between respective tracks on the disk. As a disk will often require many thousands of clock tracks across the disk, even very small phase and frequency errors in the clock track can cumulatively become very large.
U.S. Pat. No. 5,448,429 discloses another example of a system for writing clock tracks across a disk in which a written clock signal is read to provide a reference for the writing of the clock signal to a subsequent track.
U.S. Pat. No. 5,668,679 discloses another example of a method of self-servowriting a disk drive in which a clock track is written followed by spiral tracks across the disk. The clock track and the spiral tracks have missing pulses or bits and are used to locate the arm of the disk drive when the servo information is written.
In our WO-A-98/31015, the entire disclosure of which is incorporated herein by reference, there is disclosed a method and apparatus for writing clock data to a storage medium, such as a disk. The storage medium has tracks on which data can be stored. Clock data is written to a present track on the storage medium. The written clock data is read from that track. Clock data for a subsequent track on the storage medium is generated from the clock data read from said present track. The phase of the generated clock data for the subsequent track is compared with a reference timing signal and the phase of the generated clock data for the subsequent track is adjusted in accordance with said comparison. The phase adjusted clock data for the subsequent track is then written to said subsequent track. In this method and apparatus, it is again not necessary to provide a dedicated clock read/write head. As in other proposals of this type, the servo information is written to the disk using the clock track data to position the servo information very accurately. To save time, it is preferred that the servo information in a track be written alternately with the clock data in that track, i.e. in any track, a portion of clock data is written, that clock data being phase aligned with the clock data in the previous track, and then a burst of servo data is written, again appropriately phase aligned with the previous track, followed by the next portion of clock data, and so on.
However, there are a number of problems associated with using an analogue phase locked loop in a servo track writer. For example, analogue phase locked loops are extremely noise sensitive and the quality of the frequency and phase lock is difficult to determine exactly. If there is a defect on the disk where the clock track is written, analogue phase locked loops respond unfavourably to the disturbance. If the analogue signal detected through the clock head is disturbed by noise, then there is again an unfavourable response in the phase locked loop circuit. Conventional analogue phase locked loops are sometimes used in a servo track writer during the actual writing of the servo tracks in a mode in which they are not phase locked, but remain locked at the last detected frequency and phase, but this means that inevitably the loop is locked at that last frequency and phase. The analogue phase locked loop is also prone to drift and other inaccuracies in this hold mode. Also, analogue phase locked loop circuits induce errors not only into the phase but also the frequency when used in this locked mode because the last detected frequency is in principle not known. This means that analogue phase locked loops are difficult to use in a system in which one clock track is generated from a previous clock track. Finally, analogue phase locked loops require tuning to different operating frequencies due to the stability and control issues of such devices.
A further problem with the prior art is that the only connection between the two states, frequency and phase, of the servo tracks to be written and the frequency and phase of the clock track with which phase alignment is to be achieved is by means of the analogue phase locked loop, but phase and frequency are inextricably linked in an analogue phase locked loop. This makes it difficult to achieve phase alignment and frequency coherence, especially at high frequencies.
According to a first aspect of the present invention, there is provided a method of writing a servo track to a storage medium having at least one clock track thereon which is used to control the phase of the servo track written to the storage medium, the method comprising the steps of:
rotating a storage medium at a rotational frequency which is obtained from a reference frequency;
obtaining a servo pattern signal having a pattern frequency from said reference frequency;
reading a clock track from said storage medium to obtain a clock signal having a clock frequency;
adjusting the phase of the servo pattern signal relative to the clock signal so that the servo pattern signal is in phase with the clock signal; and,
writing a servo track in accordance with the servo pattern signal to the storage medium.
The motor frequency and the pattern frequency are both obtained from the same reference frequency. This greatly facilitates the achievement of frequency coherence between the clock track on the storage medium and the servo track, especially at high frequencies. The pattern frequency is in the preferred embodiment bounded to be very close to or equal to the reference frequency. The motor frequency tracks the reference frequency as accurately as possible, typically within a fraction of a percent. The maintenance of frequency coherence and of phase coherence are separated in this aspect of the invention. In the preferred embodiment, phase coherence is maintained by a phase locked loop and the reference frequency, and frequency coherence is maintained by the motor speed. The reference frequency is preferably obtained from a crystal oscillator.
The phase of the servo pattern signal is preferably digitally adjusted relative to the clock signal so that the servo pattern signal is in phase with the clock signal. The use of digital adjustment overcomes all of the problems associated with the particular use of an analogue phase locked loop experienced in the prior art. Two particular advantages are that the pattern frequency is well defined and the absolute phase of the servo pattern signal relative to the reference frequency is well defined.
The method may include the step of switching the pattern frequency between one of a plurality of discrete values thereby to adjust the phase of the servo pattern signal relative to the clock signal. Thus, the frequency of the pattern can be well bounded and very close to the intended and, in a clock copy scheme, the frequency of the track being written is not determined by the previous track and therefore does not rely on the previous track.
A digital phase locked loop is preferably used to adjust the phase of the servo pattern signal relative to the clock signal. The digital phase locked loop may include a dual-modulus prescalar, and the method may comprise the step of operating the dual-modulus prescalar to adjust the phase of the servo pattern signal relative to the clock signal.
The servo pattern signal may alternatively be obtained by the steps of passing the reference signal to two amplifiers the outputs of which are separated or shifted in phase relative to each other and summing said outputs, and the method may comprise the step of adjusting the phase of said servo pattern signal by adjusting the gain of at least one of the amplifiers. As will be discussed further below, in an example, this allows extremely small phase adjustment steps to be achieved, allowing operation of the method at very high frequencies.
According to a second aspect of the present invention, there is provided a method of writing a servo track to a storage medium having at least one clock track thereon which is used to control the phase of the servo track written to the storage medium, the method comprising the steps of:
generating a servo pattern signal having a pattern frequency; and,
reading a clock track from said storage medium to obtain a clock signal having a clock frequency;
digitally adjusting the phase of the servo pattern signal relative to the clock signal so that the servo pattern signal is in phase with the clock signal; and,
writing a servo track in accordance with the servo pattern signal to the storage medium.
As mentioned above, the use of digital adjustment overcomes all of the problems associated with the particular use of an analogue phase locked loop experienced in the prior art. The maintenance of frequency coherence and of phase coherence are separated in this aspect of the invention. In the preferred embodiment, phase coherence is maintained by a phase locked loop and a reference frequency, and frequency coherence is maintained by the motor speed.
The method may comprise the step of switching the pattern frequency between one of a plurality of discrete values thereby to adjust the phase of the servo pattern signal relative to the clock signal.
A digital phase locked loop may be used to adjust the phase of the servo pattern signal relative to the clock signal. The digital phase locked loop may include a dual-modulus prescalar, and the method may comprise the step of operating the dual-modulus prescalar to adjust the phase of the servo pattern signal relative to the clock signal.
The servo pattern signal may be obtained by the steps of passing the reference signal to two amplifiers the outputs of which are separated or shifted in phase relative to each other and summing said phase separated outputs, and the method may comprise the step of adjusting the phase of said servo pattern signal by adjusting the gain of at least one of the amplifiers.
According to a third aspect of the present invention, there is provided a method of writing a servo track to a storage medium having at least one clock track thereon which is used to control the phase of the servo track written to the storage medium, the method comprising the steps of:
generating a servo pattern signal having a pattern frequency from a reference signal by passing the reference signal to two amplifiers the outputs of which are separated or shifted in phase relative to each other and summing said outputs, the summed output being used to generate the servo pattern signal;
reading a clock track from said storage medium to obtain a clock signal having a clock frequency;
adjusting the phase of the servo pattern signal relative to the clock signal so that the servo pattern signal is in phase with the clock signal by adjusting the gain of at least one of the amplifiers; and,
writing a servo track in accordance with the servo pattern signal to the storage medium.
As mentioned above and as will be discussed in more detail below, this allows extremely small phase adjustment steps to be achieved, allowing operation of the method at very high frequencies.
The gain of the amplifiers is preferably digitally controllable.
The phase by which the outputs of the amplifiers are separated or shifted relative to each other is preferably 90xc2x0.
According to a fourth aspect of the present invention, there is provided apparatus for writing a servo track to a storage medium having at least one clock track thereon which is used to control the phase of the servo track written to the storage medium, the apparatus comprising:
a source of a reference signal having a reference frequency;
a drive circuit arranged to cause rotation of the storage medium at a rotational frequency which is obtained from the reference frequency;
a servo pattern signal generator arranged to receive the reference frequency and to output a servo pattern signal having a pattern frequency obtained from the reference frequency for use in writing a servo track on said storage medium; and,
a phase adjuster for adjusting the phase of the servo pattern signal to be in phase with a clock signal obtained from a clock track on a said storage medium.
Said phase adjuster is preferably a digital phase adjuster. The digital phase adjuster may be arranged to adjust the phase of the servo pattern signal relative to the clock signal by switching the pattern frequency between one of a plurality of discrete values.
Said phase adjuster may be a digital phase locked loop. The digital phase locked loop may include a dual-modulus prescalar. Said dual-modulus prescalar may also form part of the servo pattern signal generator.
The phase adjuster may alternatively comprise two amplifiers, a phase separator or shifter for separating or shifting the outputs of the amplifiers in phase relative to each other, a summer for summing said phase separated outputs, and a gain controller for adjusting the gain of at least one of the amplifiers thereby to adjust the phase of said servo pattern signal.
The first frequency converter may comprises a dual-modulus prescalar.
According to a fifth aspect of the present invention, there is provided apparatus for writing a servo track to a storage medium having at least one clock track thereon which is used to control the phase of the servo track written to the storage medium, the apparatus comprising:
a servo pattern signal generator for generating a servo pattern signal having a pattern frequency;
a clock track reader for reading a clock track from a storage medium to obtain a clock signal having a clock frequency; and,
a digital phase adjuster for digitally adjusting the phase of a said servo pattern signal relative to a said clock signal so that a said servo pattern signal is in phase with a said clock signal.
The phase adjuster may be arranged to switch a said pattern frequency between one of a plurality of discrete values thereby to adjust the phase of a said servo pattern signal relative to a said clock signal.
The apparatus may comprise a digital phase locked loop for adjusting the phase of the servo pattern signal relative to the clock signal. The digital phase locked loop may include a dual-modulus prescalar.
The servo pattern signal generator may comprise two amplifiers, a phase separator or shifter for separating or shifting the outputs of the amplifiers in phase relative to each other, a summer for summing said phase separated outputs, and a gain controller for adjusting the gain of at least one of the amplifiers thereby to adjust the phase of said servo pattern signal.
According to a sixth aspect of the present invention, there is provided apparatus for writing a servo track to a storage medium having at least one clock track thereon which is used to control the phase of the servo track written to the storage medium, the apparatus comprising:
a servo pattern signal generator comprising two amplifiers, a phase separator or shifter for separating or shifting the outputs of the amplifiers in phase relative to each other, and a summer for summing said phase separated outputs;
a clock track reader for reading a clock track from said storage medium to obtain a clock signal having a clock frequency; and,
a gain controller for adjusting the gain of at least one of the amplifiers thereby to adjust the phase of a said servo pattern signal relative to a said clock signal so that a said servo pattern signal is in phase with a said clock signal.
The gain of the amplifiers is preferably digitally controllable.
The phase by which the outputs of the phase shifters or phase splitter are separated or shifted relative to each other is preferably 90xc2x0.